The goal of these studies is to comprehensively characterize gender differences in Ca handling, ionic currents & beta-AR responsiveness that contribute to arrhythmogenesis, both in control and heart failure (HF), and how they are modulated by sex hormones. We have shown that VT in nonischemic HF arises primarily by nonreentrant mechanisms due to SR Ca overload activating a transient inward current (I/ti). Upregulated Na/Ca exchange (NCX) underlies greater I/ti, decreased inward rectifying K current (IK1) enhances the depolarization by I/ti and preserved beta-adrenergic receptor (beta-AR) responsiveness allows SR Ca overload. Gender differences in repolarization (prolonged QT interval & decreased K currents in females) are arrhythmogenic, but surprisingly females with HF have lower mortality and decreased arrhythmia inducibility. We hypothesize that there are gender-specific differences (due to sex hormones) in a) myocyte Ca handling, b) K currents, and c) beta-adrenergic responsiveness, and that these change differentially in HF, contributing to decreased arrhythmogenesis in HF females. Specific Aims will focus on: 1. Sex differences in Ca handling, repolarization, and beta-AR response in Ctl & HF are due to sex hormones (focused in vivo & in vitro studies (repolarization, arrhythmogenesis, Ca handling) in gonadectomized rabbits, +/- estrogen or testosterone). 2. Gender differences in Ca handling +/- HF primarily involve SR Ca transport (delta(Ca)i, SERCA, phospholamban, NCX, Ica, EC coupling,beta -adrenergic receptor (beta-AR) pathway) 3. Electrophysiological gender differences +/- HF contribute to arrhythmias (APs, ion currents (e.g. IK, Ito, IK1), channel expression, AAP duration modulates SR Ca load, in vivo +/- beta-AR stimulation). Experimental approaches will include: assessment of repolarization & arrhythmogenesis in vivo, fluorescence measurement of (Ca)i, in vitro patch clamping (voltage, current, & AP clamp), measurement of mRNA & protein (of Ca transporters & ion channel protein), assays of Ca transport function. Detailed studies in a well-characterized arrhythmogenic rabbit model of nonischemic HF (+/- gonadectomy and hormone replacement) will be mechanistically assessed by focused studies in cells & tissue from nonfailing & HF human hearts. The results will provide the foundation for novel (and possibly gender-specific) therapeutic strategies to treat arrhythmias in HF based on cellular processes that are modulated by sex hormones.